Tractor hydraulic system



Feb. 27, 1968 c, McRAE 3,370,512

TRACTOR HYDRAULIC SYSTEM Filed March 25, 1966 2 Sheets-Sheet 1 TO RAM CYLINDER TO PUMP T sump 27 I i z w a E 4 i I j C 1a /6 L .9 23

I as Fhldv 1 INV EN TOR.

&am C m aam Feb. 27, 1968 E. C. Mc 3,379,512

TRACTOR HYDRAULIC SYSTEM Filed March 25, 1966 2 Sheets-Sheet 2 TO RAM -I k CYLINDER PUMP SUMP PRIOR ART I E3355. 2 TO RAM T0 T0 CYLINDER I PUMP SUMP 2/ H I 4 I 37 36 12 l I I /3' L \35 I 1 /1 3 INVENTOR.

United States Patent Ofiflce 3,370,512 Patented Feb. 27, 1968 3,370,512 TRACTOR HYDRAULIC SYSTEM Edwin C. McRae, Rte. 1, Cusseta, Ala. 3635i) Continnation-in-part of applicatien Ser. No. 450,307, Apr. 23. 1965. This application Mar. 25, 1966, Ser. No. 537,384

12 Claims. (Cl. 91446) This application is a continuation in part of my application Ser. No. 450,307 filed Apr. 23, 1965, now Patent No. 3,322,040. The above identified application discloses the purpose of this present invention. The present invention however, diifers from that disclosed in the above identified application in that an improved cut off valve is disclosed herein which improves the sensitivity of the system thereby making it responsive to smaller increments of movement of the control valve.

The main purpose of both of these inventions is to provide a control system wherein the fluid output of a constant displacement pump is directed either to a ram cylinder or to a sump for the hydraulic fluid or is proportioned between the ram and the sump according to the position of the control valve.

This purpose is accomplished with greater sensitivity and a lower back pressure with the cut ofl valve disclosed in this present application than with the valve disclosed in the above identified application.

My improved cut off valve may be classed as a servo valve but it differs from servo valves hereto used in that its sensitivity is much improved to make the system responsive to smaller increments of movement of the control valve.

My improved cut oif valve also functions to control the back pressure in the system thereby eliminating the need for a back pressure regulating valve. This is accomplished without the use of a spring loaded valve and results in only such back pressure as is needed to actuate the servo valve at any particular time. Back pressure in the system is not needed to shift the servo valve to its unloaded position but is required to move the servo valve to the lift position. The effective area of my servo valve when moving to its lift position is several times greater than the eflective area of a conventional servo so that a lower back pressure will overcome the servo valve fric tion in my device compared to a comparable conventional servo valve.

Furthermore, the back pressure must be maintained in the conventional system sutficiently high to insure movement of the servo valve to the loaded or lift position when the oil in the system is very cold. This requires a back pressure many times higher than is necessary to insure valve movement at normal operating temperatures. With my improved servo valve the back pressure maintained in the system is only as high as is needed at any particular time so that at normal operating temperatures the back pressure is very low.

The heat build up in the hydraulic fluid is caused mainly by the dissipation of energy in the back pressure valve. The hydraulic fluid used in my system is maintained at a much lower temperature than in a conventional system because the back pressure is lower.

My invention is disclosed in the accompanying drawings, in which:

FIG.1 is a schematic diagram of the valve body of my improved hydraulic system, with the control valve and servo valve shown in their positions where the system is operating at low pressure.

FIG. 2 is a schematic diagram of a conventional control valve and servo valve and FIG. 3 is a schematic diagram of the hydraulic system shown in FIG. 2 with my improved servo valve substituted for the conventional servo valve.

Referring to the drawings, 1 have used the reference numeral 10 to indicate a valve housing. A cylindrical bore 11 is machined in the housing 1% and a control valve 12 is reciprocally mounted in the bore 11. A spring 13 urges the valve 12 against a stop 14 formed in the housing 10. An actuating head 15 is formed on the valve 12 by which the valve may be moved against the force of the spring 13.

A servo valve bore 16 is machined in the housing 10, preferably parallel to the bore 11, said bore 16 having an enlarged cylindrical section 19 in alignment with the bore 16. A servo valve 18 is reciprocally mounted in the bore 16, which valve 18 is formed integrally with a piston 19 which is reciprocally mounted in the cylinder 17.

A fluid inlet passageway 20 is drilled in the housing 10, which passageway intersects the control valve bore 11 and the enlarged section 17 of the servo valve bore 16. Fluid under pressure is thus conducted to the bore 11 and to the valve end of the cylinder 17 to thereby urge the servo valve 18 to the left at all times that fluid pressure is being applied. An 0 ring 21 is secured in the piston 19 to seal it in the cylinder 17. That portion of the cylinder 17, between the piston 19 and the valve 18 is designated chamber (X) while that portion of the cylinder 17 between the piston 19 and its outer closed end is designated chamber (Y).

A sump passageway 22 is machined in the housing 10, which passageway intersects the servo valve bore 16. A section of reduced diameter 23 is machined in that portion of the servo valve which connects the valve portion 18 with the piston 19. When the servo valve is in the position shown in FIG. 1 the valve 18 uncovers the sump passageway 20 to thereby permit fluid to flow from the inlet passageway 20 to the sump passageway 22.

A ram port 24 and a sump port 25 intersect the bore 11, in positions spaced on opposite sides of the intersection of the inlet passageway 20. The control valve 12 is provided with an elongated section 26 of reduced diam eter, one end of which, in the intermediate position of the control valve, partially uncovers the port 25 while the other end of the reduced section partially uncovers the port 26. When the valve 12 is moved fully to the right the port 25 is completely uncovered and the port 24 is closed and when the control valve 12 is moved fully to the left the port 24 is completely uncovered and the port 25 is closed.

The port 24 is connected to a ram passageway 27 which extends to a ram cylinder, not shown in the drawings. A spring urged check valve 28 is interposed in the passageway 27 so that fluid which may be forced into the ram cylinder may not return thru the passageway 27. An exhaust passageway 29 extends from the port 25 to a flutter valve bore 30 which connects the exhaust passageway 29 with the ram passageway 27. A flutter valve 31 is reciprocally mounted in the bore 30 which prevents the flow of fluid from the passageway 27 to the passage- Way 29 thru the bore 30. The sump passageway 22 intersects the bore 30 between the passageways 29 and 27. When the flutter valve 31 is moved to the left it uncovers the passageway 22 so that fluid may flow from the passageway 29 to the sump passageway and when the flutter valve is moved to the right it closes the sump passageway 22 from the passageway 29.

The function of the flutter valve 31 is described in detail in my above identified application, it being sutiicient here to state only that fluid supplied to the inlet port 20 is divided by the valve 31 so that when the control valve 12 is in its intermediate position part of the fluid is supplied to the ram passageway 27 and part to the sump passageway 29, irrespective of the pressure of the fluid in the respective passageways. When the control valve 12 is moved fully to the right all of the fluid is directed to the sump passageway 22 and when the control valve 12 is moved fully to the left all of the fluid is directed to the ram passageway 27,

It will be noted that the check valve 28 prevents the fluid in the ram cylinder from exhausting thru the passageway 27 and in order that the ram piston may be let down I have provided a bore 32 in the housing which intersects the bore 11 adjacent to the inner end of the control valve. The bore 32 connects with the ram passageway at a point between the ram cylinder and the check valve 28. The bore 32 is normally closedby the inner end of the control valve 12 but when this valve is moved fully to the right the bore 32 is uncovered to thereby permit fluid to flow from the ram cylinder into the bore 11. A conduit 33 connects the extreme inner end of the bore 11 with the sump passageway 22 so that fluid which may be discharged from the bore 32 into the bore 11 may freely flow to the sump.

It is characteristic of this hydraulic system that even though all of the fluid from the inlet passageway may be directed by the control valve to the sump passageway the flutter valve 31 functions to maintain substantially the same pressure in the inlet passageway as in the ram cylinder. It is, of course essential that when the hydraulic pump has moved the ram piston to its desired position that the pump pressure be reduced to a very low 'pressure. It is to accomplish this function that the servo valve 18 is provided. When the valve 18 is moved to the left from that shown the passageway 20 is blocked oif from the sump passageway 22 to thereby allow the pressure in the passageway 20 to build up and flow to the ram cylinder. When the valve 18 is moved back to the position shown in FIG. 1 the fluid in the passageway 20 may flow to the sump irrespective of the flutter valve 31.

In a tractor hydraulic system, where only a slight variation in the draft of an implement must actuate the control valve, it is extremely desirable that movement of the control valve thru only a few thousandths of an inch actuates the servo valve to block or unblock the inlet passageway. My improved servo valve accomplishes this function with only a fraction of the movement of the control valve that has heretofore been necessary, while still maintaining the same leakage factor in the control valve that is essential for high pressure operation.

To accomplish this I have provided a servo port 34 which intersects the bore 11 and is connected to the chamber (Y). A land 35 is provided on the valve 12 which is machined with a reduced portion 36 on one side of the land and a reduced portion 37 on the other side of the land. The reduced portion 36 is positioned between the port 34 and the reduced portion 26. The land 35 is only a few thousandths of an inch longer than the width of the port 34. The reduced portion 36 is connected to the sump by means of a drilled opening 38 in the valve 12. The opening 38 connects with the conduit 33 which is in turn connected with the sump. The reduced portion 37 is in constant communication with a high pressure passageway 39 which connects it with the passageway 27. Thus fluid under pressure in the passageway 27 is at all times supplied to the reduced portion 37 of thevalve 12.

When the control valve 12 is positioned so that the land 12 covers the port 34 the valve 12 covers about 80% of the ram port 24 and uncovers about 80% of the sump port 25. Fluid supplied to the inlet passageway 20 will at such time be directed 20% to the ram cylinder and 80% to the sump. From this position of the control valve movement of the valve 12 to the right will connect the port 34 with the reduced portion 37 to thereby permit fluid under pressure to flow into the chamber (Y) and movement of the valve 12 to the left will connect chamber (Y) with the sump.

Considerable friction is maintained between the piston 19 and the bore 16 by the O ring 21. Consequently, when the valve 12 is in the position shown in FIG. 1 the fluid pressure in the passageway 39 can move the piston to the right only enough to just partially uncover the sump passageway 22. When the valve 18 starts to uncover the passageway 22 the fluid pressure in the passageway 27 instantly drops to the point where it will move the valve 18 no further. This pressure is controlled by the friction of the O ring 21. The effective area of the piston 19 for a movement to its unloaded position will be only the area of the valve 18 as the line pressure from the inlet passageway 20 will be applied to chamber (X) and counteract all the piston area greater than the area of valve 18.

As an example, assume that the diameter of the valve 18 is .5 inch and that the diameter of the piston 19fis 1.0 inch and that the O ring produces a drag of two pounds. The area of valve 18 is .196 sq. in. and the area of piston 19 is .785 sq. in. The 2 pound drag of the O ring will thus maintain a back pressure in the system of 20 p.s.i.

If now the control valve 12 is moved to the left from that shown in FIG. 1 the chamber (Y) will be open to the sump so that the line pressure (20 psi) is only applied to the chamber (X). The effective area of the piston 19 for pressure in chamber (X) is the ditference' between the areas of the piston 19 and the valve 18. In this example this effective area is .785 sq. in., .196 sq. in. or .589 sq. in. will produce a force of l1.78 pounds to overcome the O ring drag of only 2 pounds. Thus an excess of 9.78 pounds will be applied by the back pressure to insure that the valve 18 starts to move to its closed or lift position.

The O ring drag may be less or more than two pounds but the ratio of the eflective diameters of the piston 19 for unloading and producing lift will always insure ample back pressure to actuate the servo valve to the left position. An increase in O ring drag will cause a higher back pressure to be maintained which will give a net eflective force on the piston greater than in the example described herein In actual practice the drag of such an O ring is non mally less than /2 pound but whatever it is the effective force for moving the servo valve to lift position is always several times that required to move it to the unloading position.

The actuation of my improved servo valve has been explained in detail to bring out that the back pressure in my system when the oil is at operating temperature is very low whereas the back pressure in the conventional system must be maintained high enough at all times to move the servo valve under the most adverse conditions that is when the oil is very cold.

The conventional tractor hydraulic system is shown in FIG. 2 from which it will be seen that the inlet passageway 20 is connected to a recessed portion 40 on the control valve and also to the chamber (X). The port 34 is covered by a land 44 on the valve 12 so that movement of the valve 12 will cause fluid from passageway 20 to flow into chamber (Y) or permit fluid in chamber (Y) to discharge to the sump. The inlet passageway 20 is connected to a low pressure line 41 thru a spring loaded back pressure valve 42. The servo valve 18 is recessed at 43 and the sump passageway 22 is positioned between the line 41 and the adjacent end of the enlarged cylinder 17.'

The recessed portion 43 is long enough so that when the piston 19 is in its extreme left hand position, shown in FIG. 2, this recess connects the line 41 with the sump passageway 22. The inlet passageway 20 is maintained with a constant back pressure governed only by the resistance of the back pressure palve 42. When the pistonfrom the land 44 to a point beyond the intersection of the inlet passageway 20. When the valve is in the position shown in FIG. 2 fluid pressure will be applied only to the chamber (X), the chamber (Y) being at such time open to the atmosphere. The pressure in chamber (X) will thus move the valve to its unloaded position.

When the control valve is moved to the left the reduced section 40 on the control valve admits oil into chamber (Y) to thereby force the piston 19 to the right over an effective area equal to the area of valve 18. When the valve has completely blocked off the sump passageway 22 the oil is forced to the ram cylinder to start the lift.

It should be mentioned at this point that even a slight leakage of fluid from the bore 32. will cause the ram piston to slowly drift down under the Weight of the implement when the system is in its unloaded position. Such drift down will cause the valve 12 to move very slowly to the left. The servo valve 18 must function to periodically engage the lift to correct for such drift down. This has always been a critical function of such valves. There is always sufficient movement of the ram piston during lift to insure a prompt cut 0E and, of course, when the control valve is manually moved its motion is relatively fast to insure a prompt shift of the servo valve to its loaded position. However, to correct for drift down the shift of the servo must be accomplished with a very slow movement of the control valve.

In the conventional system, shown in FIG. 2, the servo valve 18 must be moved thru its complete stroke to entirely cover the sump passageway 22 before lift can occur. Leakage from the reduced portion 45 around the land 44 will seep into the chamber (Y) and just gradually move the servo valve. This is especially true where only a small overlap of the land is provided because pressure will not build up behind the piston 19 until the land 44 fully closes the port 34 to the atmosphere. Consequently land 44 must be long enough to close port 34 before leakage from 45 starts to move the piston.

In the applicants device the servo valve need be moved only a few thousandths of an inch to start building up the back pressure whereas in the conventional system the servo valve must be moved thru nearly its full stroke before the pressure build up will fully close the servo valve. This is one reason why the overlap of the applicants land 34 may be made considerably less than the land 44.

Another reason why applicants servo valve is moved sooner than the conventional valve is that the effective pressure on the piston 19 in the applicants device is several times greater for moving the servo valve to its lift position than on the conventional valve. In the illustration described the conventional servo for movement to lift position has an effective area of only .196 sq. in. whereas the same size valve in the applicants device has an effective area of .589 sq. in.

The above mentioned difierence has been brought out because it is absolutely essential that the servo valve be moved fast enough When correcting for drift down to prevent the valve from hanging up. Hanging up is that condition where the servo valve closes off the inlet port just enough to build up a pressure slightly more than ram pressure but not sufficiently high to actually move the ram piston. In such case the pump operates continuously at ram pressure but the ram doesnt lift.

FIG. 3 of the drawings has been shown to illustrate a system which is conventional in every way except that my improved servo valve has been substituted for the conventional servo valve. in this view the servo valve is shown in its unloaded position where inlet pressure is applied to both chambers (X) and (Y). It now the control valve is moved to the left, the lift position, the land 35 closes off the chamber (Y) from the inlet pressure and connects it with the sump. The back pressure in the inlet passageway 29 is now applied only to the chamber (X) and as the effective area of the chamber (X) is several times that of chamber (Y) the valve 13 is positively moved by a back pressure which only balances the valve friction during the unloading movement. Once the valve starts to move to the left the pressure immediately builds up in the inlet passageway, which pressure accelerates the closing movement of the servo. In practice, once the servo valve starts to move to its lift position the build up in line pressure almost instantly completes the movement.

Movement of the ram piston starts the control valve 12 moving to the right and when it has almost reached its out off point high pressure oil from the reduced portion 37 will start to leak into the chamber (Y) around land 35. Since the overlap of land 35 is less than that of the conventional land 44 there will be oil leakage around th land 35 but this leakage will flow into the chamber (Y) not out of this chamber as in the conventional system. This leakage must build up a pressure in chamber (Y) to 75% of the line pressure before it will move the servo valve to its unloaded position. Even though the land 35 has little or no overlap on the port 34 leakage cannot build up any appreciable pressure in chamber (Y) until the inner end of land 35 completely blocks off chamber (Y) from the reduced portion 36. When this occurs the valve 18 is quickly moved to its unloaded position and further movement of the valve 12 ceases. The pressure in the entire system then drops to the negligible amount controlled by the O ring friction. No further leakage of oil into chamber (Y) occurs because even a hair line overlap of the port 34 by the inner end of land 35 will prevent such leakage under the low back pressure. The outer end of land 35 has at this time not quite uncovered the port 34 but no leakage into or out of chamber (Y) occurs because chamber (Y) is at such time subject only to back pressure.

If now the valve 12 is moved to the left by only a thousandth of an inch the pressure in the chamber (Y) starts to drop. When this pressure drops to 75% of back pressure the back pressure in chamber (X) starts to move the valve 18 to close off the sump passageway 22 as previously described.

With the conventional valve, shown in FIG. 2 the control valve must be moved thru twenty to thirty thousandths of an inch to actuate the servo valve, Whereas with my improved construction movement of the control valve thru only a few thousandths of an inch will actuate the servo valve. This is an important advantage of my improved servo valve where small increments of movement of the ram piston are required and where small variation in the draft of an implement must operate the servo valve.

While I have shown in FIG. 1 the combination of my improved servo valve with my mechanism for varying the rate of lift independently of the pump displacement, it will be apparent that my improved servo valve when installed in the conventional system will increase the sensitivity of the system and produces smaller increments of movement of the ram piston. However the operation will still be abrupt because of the rapid acceleration of the ram piston when the full output of the pump is forced into the ram cylinder.

Referring to FIG. 1 of the drawings, it will be apparent that in as much as the servo valve is moved to its unloaded position before the valve 1.2 has completely closed off the port 24 it is not necessary that the port 24 ever be fully closed. In commercial practice, the portion of the valve 12 which covers the port 24 may be made ten to fifteen thousandths of an inch smaller than the bore 11 so that even though the valve 12 completely covers the bore 12 at the time that the servo valve is moved to its unloaded position sufiicient fluid will at such time flow into the ram passageway to keep moving the ram piston until the servo valve is unloaded.

It will also be apparent that when the control valveis in either its slow or hi h speed rate of lift position a relatively small opening thru the port 24 will not unduly raise the pump pressure above the ram pressure. Only a small drop in pressure from the inlet passageway to the ram passageway is necessary to operate the flutter valve. Consequently, a fixed orifice between the inlet passageway and the ram passageway may be provided in place of the port 24 and the associated portion of the valve 12.

The claims in this case which are limited to a control valve which closes the inlet passageway to the ram passageway should be construed as covering any restriction which reduces the opening between the inlet passageway and the ram passageway sufficiently to insure slow speed operation of the ram piston at the time the servo valve is moved to its unloaded position.

The advantages of my system may be listed as follows:

The increase in sensitivity of my system permits the valve to be operated by the draft resistance of light implements The provision of smaller increments of ram piston movement provides for better implement adjustment in the ground.

The variable lift rate provided results in smoother starting and stopping when the implement is being raised and also permits an implement to be slowly raised as is desirable in operating a blade for grading.

The elimination of the conventional back pressure valve results in a cost reduction.

The fact that the average back pressure maintained in my system is much lower than that required in the conventional system results in a lower operating temperature for the oil used in the system. The reduction of the power loss resulting from the lower back pressure in my system is a further advantage.

Some changes may be made in the arrangement and design of my improved hydraulic system without departing from the scope of my invention and it is my intension to cover by my claims such changes as may reasonably be included within the scope thereof.

I claim as my invention:

1. In a hydraulic system for controlling the 'flow of fluid to a ram cylinder, a valve housing having an inlet passageway therein for conducting fluid to said housing and having a sump passageway therein for conducting fluid from said housing to the sump for said system and having a ram passageway therein which connects said inlet passageway to said ram cylinder, said housing having a servo valve bore therein which terminates in an enlarged servo valve cylinder axially aligned with said bore, a closure for the outer end of said cylinder, the area of said cylinder being substantially greater than the area of said servo valve bore, and being positioned in said housing so that said inlet passageway intersects said cylinder adjacent to said servo valve bore with said sump passageway intersecting said servo valve bore at a point spaced from said cylinder, a servo valve reciprocally mounted in said servo valve bore, said servo valve having a valve section thereon which forms a closure between said cylinder and said sump passageway and having a piston section which closely fits said cylinder and having an intermediate section of smaller area than said valve section connecting said valve section with said piston, said servo valve at one end of its stroke closing said inlet passageway to said sump passageway and at the other end of its stroke opening said cylinder to said sump passageway so that fluid from said inlet passageway may flow through said cylinder and along said intermediate section to said sump passageway, and a control valve for said system which in one position admits fluid from said inlet passageway to said cylinder at a point between the outer end of said cylinder and said piston and in another position exhausts fluid from said cylinder to said sump, said control valve being supported for reciprocation in said housing, said control valve being provided with a land thereon which in an intermediate position of said control valve forms a closure for a port which leads from said land to the closed end of said cylinder, the portion of said control valve on one side of said land being in permanent communication with said inlet passageway and the portion of said control valve on the other side of said land being in permanent communication with said sump passageway. V

2. A hydraulic system for controlling a hydraulically actuated device, said system comprising an inlet conduit adapted to receive fluid under pressure, an exhaust conduit adapted to discharge fluid to a sump, first passage 7 means interconnecting said inlet conduit with said exhaust conduit, a servo valve for controlling the flow through said first passage means, a piston fixed to said servo valve for moving said servo from a closed position wherein fluid flow through said first passage means from said inlet conduit to said exhaust conduit is obstructed to an open position wherein fluid flow from said inlet conduit to said exhaust conduit is unobstructed, and means including a control valve for selectively conducting line pressure to one side of said piston for moving said servo V valve from its closed position toward its open position a sufilcient distance to maintain a pressure in said inlet conduit substantially greater than the pressure in said exhaust conduit, said last named means comprises second fluid passage means exposed at one end to the pressure in the inlet conduit and third fluid passage means exposed at one end to sump pressure, said control valve being supported for movement between a neutral position in which the one side of the piston is exposed to the pressure in said second fluid passage means and an actuating position in which said one side of said piston is exposed to the pressure in said third passage means, and further including fourth fluid passage means interconnecting said inlet con- .duit with the other side of said piston, said one side of said piston having a significantly greater effective area exposed to fluid pressure than said other side, said servo valve being moved toward its closed position when the force acting upon said other side of said piston exceeds the resistance of said piston to move upon a predetermined decrease in pressure acting on said one side of said piston for obstructing fluid communication between said inlet conduit and said exhaust conduit for increasing the back pressure acting on said other side of said piston through said fourth fluid passage means for accelerating the closing of said servo valve.

3. A hydraulic system as set forth in claim 2 further including fifth fluid passage means interconnecting the inlet conduit with the hydraulically actuated device for actuating the device when fluid communication from said inlet conduit to the exhaust conduit is restricted by closure of said servo valve.

4. A hydraulic system as set forth in claim 2 further including means for resisting the movement of the servo valve from its closed position to its open position for establishing a predetermined back pressure in the inlet conduit when both sides of said piston are exposed to the pressure in said inlet conduit.

5. A hydraulic system for controlling a fluid actuated formed in said valve housing, a fluid inlet conduit formed in said valve housing and intersecting said control valve bore, one end of said fluid inlet conduit being adapted to be connected to a source of fluid under pressure, an exhaust conduit formed in said value housing and intersecting control valve bore at a point spaced from the intersection of said inlet conduit with said valve bore, one end of said exhaust conduit being adapted to be connected to a fluid sump, first passage means formed in said valve housing and intersecting said valve bore at a point spaced from the intersection of said inlet conduit and said exhaust conduit with said valve bore, one end of said first passage being adapted to be connected to the fluid actuated device, a control valve having first and second spaced lands formed thereon and supported for reciprocation within said control valve bore, said first land cooperating with said first fluid passage for controlling the flow from said valve bore to said first fluid passage, said second land cooperating with said exhaust passage for controlling the flow from said valve bore to said exhaust passage, and a modulating valve supported within said valve housing for modulating the flow through said exhaust conduit, said modulating valve being movable from a first extreme position wherein flow through both said first passage and said exhaust conduit is unobstructed to a second extreme position wherein it obstructs flow through said exhaust conduit while still leaving said first passage unobstructed.

6. A hydraulic system as set forth in claim further including second passage means interconnecting the inlet conduit with the exhaust conduit, a servo valve for controlling the flow through said second passage means, a piston fixed to said servo valve for moving said servo valve from an open position wherein fluid flows through said second passage means to a fully closed position wherein fluid flow from said inlet conduit to said exhaust conduit through said second passage means is obstructed, and means including the control valve for selectively conducting the pressure in said inlet conduit to one side of said piston for moving said servo valve from its closed position toward an open position a suflicient distance to maintain a pressure in said inlet conduit substantially greater than the pressure in said exhaust conduit.

7. A hydraulic system as set forth in claim 6 wherein the last named means includes third fluid passage means interconnecting the first passage means at a point downstream of the modulating valve with the one side of the piston, fourth fluid passage means interconnecting said one side of said piston with the sump and third land means on the control valve spaced from the first and second lands for selectively controlling the communication between said third passage means and said fourth passage means with said one side of said piston.

8. A hydraulic system as set forth in claim 7 further including means for exposing the other side of the piston to the pressure in the inlet conduit, the eifective area of the one side of said piston being greater than the effective area of said other side of said piston.

9. A hydraulic system as set forth in claim 8 further including fifth fluid passage means interconnecting the fluid actuated device with the sump and fourth land means on the control valve adapted to control the flow through said fifth passage means.

10. A hydraulic system for controlling a hydraulically actuated device, said system comprising an inlet conduit adapted to receive fluid under pressure, an exhaust conduit adapted to discharge fluid from said inlet conduit to a sump, an actuated device conduit adapted to conduct fluid from said inlet conduit to said actuated device, first passage means interconnecting said inlet conduit with said exhaust conduit, a servo valve for controlling the flow through said first passage means, a piston fixed to said servo valve for moving said servo valve from a closed position wherein fluid flow through said first passage means from said inlet conduit to said exhaust conduit is obstructed to an open position wherein fluid flow from said inlet conduit to said exhaust conduit is unobstructed, a modulating valve interconnecting said inlet conduit and said exhaust conduit, and a control valve for actuating said modulating valve to proportion the flow of fluid from said inlet conduit to said actuated device conduit from full flow to substantially 20% flow, said control valve selectively conducting line pressure to one side of said piston for moving said servo valve from its closed position to its open position when said control valve is moved from its full flow position to said 20% flow position.

11. A hydraulic system as set forth in claim 10 wherein said servo valve is moved by said line pressure from its closed position toward its open position a suflicient distance to maintain a pressure in said inlet conduit substantially greater than the pressure in said exhaust conduit.

12. A hydraulic system for controlling a hydraulically actuated device, said system comprising an inlet conduit adapted to receive fluid under pressure, an exhaust conduit adapted to discharge fluid to a sump, first passage means interconnecting said inlet conduit with said exhaust conduit, a servo valve for controlling the flow through said first passage means, a piston fixed to said servo valve for moving said servo from a closed position wherein fluid flow through said first passage means from said inlet conduit to said exhaust conduit is obstructed for increasing the pressure in said inlet conduit to an open position wherein fluid flow from said inlet conduit to said exhaust conduit is unobstructed, means for continuously applying pressure from said inlet conduit to one side of said piston, the pressure on said one side of said piston tending to move said servo valve toward its closed position, and means including a control valve for operating said servo valve, said control valve having a neutral position for conducting fluid from said inlet conduit to the other side of said piston, said other side of said piston having a significantly greater efiective area than said one side of said piston for moving said piston and said servo valve toward the open position of said servo valve when both sides of said piston experience the pressure in said inlet conduit, said control valve being movable from said neutral position to an actuating position for exposing said other side of said piston to said sump, the pressure in said inlet conduit acting on said one side of said piston being effective to move said piston and said servo valve toward the closed position of said servo valve when said other side of said piston is exposed to said sump for increasing the pressure in said inlet conduit, the increase in the pressure in said inlet conduit acting upon said one side of said piston being eiiective to accelerate the closing of said servo valve.

References Cited UNITED STATES PATENTS 2,799,251 7/ 1957 Newgen 91-384 2,847,030 8/1958 McRae 137596.13 3,088,283 5/1963 Furia et a1. 91384 3,103,147 9/1963 Peras 9l384 MARTIN P. SCHWADRON, Primary Examiner. PAUL E. MASLOUSKY, Examiner, 

1. IN A HYDRAULIC SYSTEM FOR CONTROLLING THE FLOW OF FLUID TO A RAM CYLINDER, A VALVE HOUSING HAVING AN INLET PASSAGEWAY THEREIN FOR CONDUCTING FLUID TO SAID HOUSING AND HAVING A SUMP PASSAGEWAY THEREIN FOR CONDUCTING FLUID FROM SAID HOUSING TO THE SUMP FOR SAID SYSTEM AND HAVING A RAM PASSAGEWAY THEREIN WHICH CONNECTS SAID INLET PASSAGEWAY TO SAID RAM CYLINDER, SAID HOUSING HAVING A SERVO VALVE BORE THEREIN WHICH TERMINATES IN AN ENLARGED SERVO VALVE CYLINDER AXIALLY ALIGNED WITH SAID BORE, A CLOSURE FOR THE OUTER END OF SAID CYLINDER, THE AREA OF SAID CYLINDER BEING SUBSTANTIALLY GREATER THAN THE AREA OF SAID SERVO VALVE BORE, AND BEING POSITIONED IN SAID HOUSING SO THAT SAID INLET PASSAGEWAY INTERSECTS SAID CYLINDER ADJACENT TO SAID SERVO VALVE BORE WITH SAID SUMP PASSAGEWAY INTERSECTING SAID SERVO VALVE BORE AT A POINT SPACED FROM SAID CYLINDER, A SERVO VALVE RECIPROCALLY MOUNTED IN SAID SERVO VALVE BORE, SAID SERVO VALVE HAVING A VALVE SECTION THEREON WHICH FORMS A CLOSURE BETWEEN SAID CYLINDER AND SAID SUMP PASSAGEWAY AND HAVING A PISTON SECTION WHICH CLOSELY FITS SAID CYLINDER AND HAVING AN INTERMEDIATE SECTION OF SMALLER AREA THAN SAID VALVE SECTION CONNECTING SAID VALVE SECTION WITH SAID PISTON, SAID SERVO VALVE AT ONE END OF ITS STROKE CLOSING SAID INLET PASSAGEWAY TO SAID SUMP PASSAGEWAY AND AT THE OTHER END OF ITS STROKE OPENING SAID CYLINDER TO SAID SUMP PASSAGEWAY SO THAT FLUID FROM SAID INLET PASSAGEWAY MAY FLOW THROUGH SAID CYLINDER AND ALONG SAID INTERMEDIATE SECTION TO SAID SUMP PASSAGEWAY, AND A CONTROL VALVE FOR SAID SYSTEM WHICH IN ONE POSITION ADMITS FLUID FROM SAID INLET PASSAGEWAY TO SAID CYLINDER AT A POINT BETWEEN THE OUTER END OF SAID CYLINDER AND SAID PISTON AND IN ANOTHER POSITION EXHAUSTS FLUID FROM SAID CYLINDER TO SAID SUMP, SAID CONTROL VALVE BEING SUPPORTED FOR RECIPROCATION IN SAID HOUSING, SAID CONTROL VALVE BEING PROVIDED WITH A LAND THEREON WHICH IN AN INTERMEDIATE POSITION OF SAID CONTROL VALVE FORMS A CLOSURE FOR A PART WHICH LEADS FROM SAID LAND TO THE CLOSED END OF SAID CYLINDER, THE PORTION OF SAID CONTROL VALVE ON ONE SIDE OF SAID LAND BEING IN PERMANENT COMMUNICATION WITH SAID INLET PASSAGEWAY AND THE PORTION OF SAID CONTROL VALVE ON THE OTHER SIDE OF SAID LAND BEING IN PERMANENT COMMUNICATION WITH SAID SUMP PASSAGEWAY. 